As shown in FIG. 3, cutting portion of a drill D1 comprises, in general, a chisel edge 26, a major cutting edge 1 extending linearly outwards from a chisel edge corner 26a, a major flank 2 extending in a backward direction of drill rotation from the major cutting edge 1, and a rake face 3 extending from the major cutting edge 1 in the shape of a grooved surface.
Conventionally, for the drilling of inorganic substances such as ceramics and other materials that are hard to cut (hereinafter referred simply to "work-piece"), a carbide drills having ultra-hard alloys as cutting edge material, a drill electroplated with ultra-hard cutting grains S, such as diamond, to the major cutting edge 1, or the like (hereinafter referred simply to "drill"), were used.
In such conventional drill D1, the major flank 2 was formed as descending slope in a backward direction of drill D1 rotation from the major cutting edge 1. When drill the work-piece, the chisel edge 26 comes into contact with the work-piece at first, then the major cutting edge 1 begins to cut the work-piece and continues thereafter with the major cutting edge 1 only.
As a result, the major cutting edge 1 tended to fatigue and the drill must be exchanged often during the drilling. In particular, in case of a drill electroplated with the ultra-hard cutting grains S to its major cutting edge 1, the ultra-hard cutting grains S tended to drop off due to its linear configuration. Also adhering the ultra-hard cutting grains S to the major cutting edge 1 was very difficult.
In the meantime, during the drilling operation, a rotating drill is pressed against the work-piece with a fixed pressure until drilling is completed supplying a coolant for lubrication and cooling. Inevitably, the rotation of the drill was braked due to friction between the drill and the work-piece. Then, the drilling operation required long time and the work-piece was sometimes damaged or broken.
The continuous contact of the rotating drill with the work-piece made it difficult to penetrate the coolant into the drilled hole. Consequently, the cutting portion of the drill and the work-piece are not adequately cooled but are heated to a high temperature. It caused decreasing of durability of the drill and deterioration and damage of the work-piece.